The present invention relates generally to centrifugal blowers. More specifically, the present invention is directed to a method and housing configuration for attenuating the sound caused by blade pass in centrifugal blowers.
The need to move large quantities of air is ever present in heating, ventilating and air conditioning appliances. To meet these air handling requirements, a variety of fans and blowers are most often employed. Of the various types of blowers, centrifugal blowers are the most widely used because they can effectively move large or small quantities of air over a wide range of pressures. Unfortunately, one of the drawbacks to these types of blowers is that the blowers generate unwanted sound (noise;) that can be a distraction or annoyance, or more seriously, can impose health and safety risks.
The noise from centrifugal blowers is a primary superposition of discrete frequency noise (pure tones) at the impeller or blade pass frequency. The origin of these discrete tones stems from two sources. First, each time a blade passes a point in space, a pressure fluctuation is created at the blade passing frequency due to the displacement of air. Second, as the blades pass the cut off point in the scroll section of the housing, abrupt pressure changes or pulses also occur at the blade passing frequency.
The predominant method of attenuating the sound generated by centrifugal blowers has been to place a system of filters and/or silencers at the inlet or outlet of the blower. Although these types of silencing devices can reduce the sound emanating from the blower, the use of a silencer or filter results in several drawbacks. First, silencers and/or filters add to the overall cost of the air handling system since silencers and filters are additional parts that need to be properly selected and engineered into the system to be effective and need to be maintained/replaced when necessary. Second, silencing devices take up unnecessary space near the blower housing. Finally, silencing devices can affect the overall performance of the system because the silencing devices can become clogged with dust particles and other forms of debris.
Thus, due to the widespread use of blowers many of today""s air handling applications, a need exists for a more efficient and reliable method of attenuating unwanted sound (noise) created by blade pass in an operating blower.
The present invention relates to an apparatus and method for attenuating noise caused by blade pass in centrifugal blowers. The centrifugal blower of the present invention includes a blower housing that encloses a rotating impeller for creating a flow of air out of an outlet opening formed by the blower housing. An electric motor is mounted onto the blower housing such that the motor shaft rotates the impeller within the blower housing.
As the impeller rotates within the blower housing, the blades of the impeller pass by a cut off formed along the interior of the blower housing between the scroll section of the blower housing and an integrally formed exhaust section.
In accordance with the invention, an angled cut off portion is formed along the intersection between the tubular exhaust section and the scroll section in order to disrupt the pressure fluctuations created by the orientation of the cut off and the impeller blades. Specifically, the angled cut off portion is formed by a protruding molded area having an irregular outer surface that varies the angle between the impeller blades and the cut off edge.
In addition to the angled cut off portion, the blower housing of the present invention includes at least one sound cavity that extends outward from the outer wall of the exhaust section. The sound cavity creates an open space that attenuates the sound created by the rotating impeller within the blower housing. In the preferred embodiment of the invention, a single sound cavity is positioned between the outlet opening formed in the exhaust section and the scroll section of the blower housing. However, it is contemplated that a pair of sound cavities could be utilized on opposite sides of the exhaust section to further dampen the noise created by the rotating impeller.
An important aspect of the invention is that the combination of the sound cavities and the angled cut off provide sound attenuation without effecting the blower performance. A further advantage of the present invention is to provide an improved blower housing and design a method that attenuates noise associated with impeller blade pass without requiring additional components.